﻿using Prism.Common;
using Prism.Properties;

namespace Prism.Navigation.Regions.Behaviors;

/// <summary>
/// Behavior that synchronizes the <see cref="IRegion.Context"/> property of a <see cref="IRegion"/> with
/// the control that hosts the Region. It does this by setting the <see cref="Xaml.RegionManager.RegionContextProperty"/>
/// Dependency Property on the host control.
///
/// This behavior allows the usage of two way databinding of the RegionContext from XAML.
/// </summary>
public class SyncRegionContextWithHostBehavior : RegionBehavior, IHostAwareRegionBehavior
{
    private const string RegionContextPropertyName = "Context";
    private VisualElement _hostControl;

    /// <summary>
    /// Name that identifies the SyncRegionContextWithHostBehavior behavior in a collection of RegionsBehaviors.
    /// </summary>
    public static readonly string BehaviorKey = "SyncRegionContextWithHost";

    private ObservableObject<object> HostControlRegionContext => RegionContext.GetObservableContext(_hostControl);

    /// <summary>
    /// Gets or sets the <see cref="VisualElement"/> that the <see cref="IRegion"/> is attached to.
    /// </summary>
    /// <value>
    /// A <see cref="VisualElement"/> that the <see cref="IRegion"/> is attached to.
    /// This is usually a <see cref="VisualElement"/> that is part of the tree.
    /// </value>
    public VisualElement HostControl
    {
        get
        {
            return _hostControl;
        }
        set
        {
            if (IsAttached)
            {
                throw new InvalidOperationException(Resources.HostControlCannotBeSetAfterAttach);
            }
            _hostControl = value;
        }
    }

    /// <summary>
    /// Override this method to perform the logic after the behavior has been attached.
    /// </summary>
    protected override void OnAttach()
    {
        if (HostControl != null)
        {
            // Sync values initially.
            SynchronizeRegionContext();

            // Now register for events to keep them in sync
            HostControlRegionContext.PropertyChanged += RegionContextObservableObject_PropertyChanged;
            Region.PropertyChanged += Region_PropertyChanged;
        }
    }

    void Region_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
    {
        if (e.PropertyName == RegionContextPropertyName)
        {
            if (Xaml.RegionManager.GetRegionContext(HostControl) != Region.Context)
            {
                // Setting this Dependency Property will automatically also change the HostControlRegionContext.Value
                // (see RegionManager.OnRegionContextChanged())
                Xaml.RegionManager.SetRegionContext(_hostControl, Region.Context);
            }
        }
    }

    void RegionContextObservableObject_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "Value")
        {
            SynchronizeRegionContext();
        }
    }

    private void SynchronizeRegionContext()
    {
        // Forward this value to the Region
        if (Region.Context != HostControlRegionContext.Value)
        {
            Region.Context = HostControlRegionContext.Value;
        }

        // Also make sure the region's DependencyProperty was changed (this can occur if the value
        // was changed only on the HostControlRegionContext)
        if (Xaml.RegionManager.GetRegionContext(HostControl) != HostControlRegionContext.Value)
        {
            Xaml.RegionManager.SetRegionContext(HostControl, HostControlRegionContext.Value);
        }
    }
}
